Rocket nozzles



July 14, 1970 A SHALER 3,520,478

ROCKET NOZZLES Filed June 6, 1966 If. G F O O 0 O I H... I \0 J G O I I111. III.

INVENTOR AMOS J. 579141.51?

ATTORNEYS.a

United States Patent US. Cl. 239265.15 5 Claims ABSTRACT OF THEDISCLOSURE Rocket nozzles and nozzle inserts of the autotranspirationalcooling type comprise a porous body of refractory material, for example,graphite, refrectory metal, or a refractory metal carbide. Hotpropellant gases pass through a bore which provides a flame-contactingface. The outside of the body forms its back face. In accordance withthe invention the body is provided with an array of closely spacedpassages, or holes, radially disposed relative to the nozzle axis; theyextend from the flame face to the back face. The porosity and passagesare infiltrated in the region of said back face to a depth betweenonetenth and one-quarter of the thickness of the body at its throat witha substance that is solid, non volatile and of low coeflicient ofthermal expansion at the operating temperature of the back face of theinsert and which acts to reinforce the body against stresses due to thedifferential expansion during firing, and the passages and remainingporosity are then infiltrated with a metal which melts above about 1000F., exerts high vapor pressure at the flame temperature and isnon-reactive with the refractory body at temperatures, the latterreaches in service.

This invention relates to rockets, missiles, and other jet propulsiondevices, and more particularly to the nozzles and nozzle inserts of suchdevices which are subjected to extremely high temperatures and otherdamaging conditrons.

The propellants used with devices of the type mentioned developcombustion products at temperatures of the order ofat least about 5000F., and recently the trend has been toward the development of stillhigher temperatures, approaching 7000 and even 7500 F. Such temperaturesimpose severe requirements upon the materials used. First, it isrequisite not only that the nozzle insert be refractory in the sensethat in operation it will not melt or soften or erode to an extent suchthat would result in failure, but also that it will not crack or distortunder the stresses created by the flame.

The refractory materials available for such purposes are generally poorconductors of heat, and large temperature gradients, which may amount toseveral thousand degrees Fahrenheit between the flame face and the back,or outer face, are created in a few seconds after firing. This tends tocreate thermal stress conditions which may, and commonly will, causecracking, and even failure, of the nozzle part under the stress patterncreated by the thermal conditions and by the flame pressure. Also, thematerial at the flame surface must resist erosion and corrosion by thecombustion gases and the liquid and solid combustion products present inthe flame. By resist is meant that the erosion and corrosion must bekept below a maximum acceptable value, which desirably is as low as asurface recession of a few thousandths of an inch per minute under flametemperatures of the order of 6000 to 7000 F., flame pressures of theorder of 1000 p.s.i., and flame compositions which include substantialquantities of, for example, carbon dioxide, water vapor, hydrogenchloride, and oxides of aluminum and other metals.

Obviously, one way of minimizing the foregoing objectionable consequenceof the flame temperature and pres- "ice sure is to provide some means ofcooling the nozzle or its insert, and various ways of doing so have beenproposed and tried. One of the most promising of these proposals is byautotranspirational cooling, the principle of which is well known. Itinvolves a nozzle or insert comprising a coherent skeleton of refractorymaterial having a multiplicity of interconnected pores of finedimensions which have been infiltrated with a solid phase which meltsunder the flame heat to produce liquid of high vapor pressure; the vaporformed then moves through the pores into the flame. Heat is absorbed bythis material while it is solid, liquid, and gaseous during the time itis being heated to the temperature of the flame, as well as when it isundergoing a transformation, such as in melting and vaporizing, andoften, if the infiltrant is a compound, in dissociating it. In addition,cooling is provided by the reduction of the convective coeflicient ofheat transfer from the flame through the boundary layer, by its constantdilution with the outpouring vapor of the infiltrant.

Such autotranspiring inserts may be provided by carboas, graphites, bysuch carbides as monotantalum carbide or mixed carbides of tantalum orzirconium and hafnium, or they may be made from refractory metals suchas tungsten, and the pores infiltrated with, for example, silver orcopper, or a combination of one of these with a lower-boiling materialsuch as lead, polyethylene, or a fluorocarbon polymer, which under theinfluence of the heat of the flame passing through the nozzle will meltand be vaporized through the pores into the flame. The transformation ofthe infiltrant from its solid phase condtion to a hot vapor phaseresults in the absorption of heat from the porous material withconsequent reduction in the rate of temperature increase of the insertwhereby operation is permitted for a sufficient period of time in thepresence of the flame, the temperature of which is such that withoutthis cooling effect the insert would fail by softening, cracking,melting, erosion, or a combination of these factors. Thusautotranspirational cooling is capable of maintaining the integrity ofthe nozzle and its insert for the length of time required for themission of the device.

Such autotranspirational articles may be made and infiltrated bywell-known powder-metallurgy methods, the size and extent of theinterconnected pores being controllable by the particular practiceapplied.

This cooling principle has been applied to tungsten inserts infiltratedwith silver. Experience has shown that such inserts leave much to bedesired because of objectionable factors and disadvantages. In the firstplace, the temperature of the insert may become so rapidly elevated atthe flame surface, that the tungsten skeleton of that region will sinterto a substantially impenetrable skin so that the pores beneath it are nolonger open to the flame surface. If, then, the evaporating phase isstill substantially solid or liquid it will expand thermally faster thanthe tungsten skeleton with creation of pressure causing internal cracksunder the sintered skin which is lifted by the pressure to formblisters. Blisters are objectionable because the smoothness of the flamesurface is interrupted which prevents ideally smooth passage of theflame with reduction of nozzle efliciency.

Secondly, after a period of service the level of the infiltrant fromwhich evaporation is taking place has receded into the insert far awayfrom the flame surface, and because the resistance of the fine pores tothe movement of the infiltrant vapor through them increases rapidly astheir effective length increases, the quantity of vapor entering theflame per unit of time may fall off to an undesirably low value.

Thirdly, if the evaporating phase expands thermally faster than thetungsten skeleton, which is commonly the case, a portion of theinfiltrant may be extruded from the flame surface as a result of thestress thus created and enter the flame as liquid or incompletelyvaporized liquid with consequent impairment of the intended coolingeffect.

It is among the objects of this invention to provide nozzles and nozzleinserts of refractory materials, as that term is used herein, for jetpopulsion devices, such as rockets and missiles, which provide theadvantages of autotranspirational cooling while suppressing theobjectionable factors that have been encountered withautotranspirational cooling.

Another object is to provide nozzles of the type contemplated by theforegoing object in the use of which erosion is minimized and internallyinduced stresses are resisted, and which also are strengthened againststresses created at the outer, or back face, that is, the face oppositethe flame face.

A further object is to provide nozzles in accordance with the foregoingobjects and which have the porosity adjacent the back face occupied by astrength-reinforcing infiltrant, particularly one that is non-volatile,and having the pores adjacent the flame face occupied by one, or morethan one, vaporizable infiltrant.

Yet another object is to provide such nozzle inserts which provideimproved movement of vaporizable infiltrant to the flame face and reducethe tendency of the nozzle to crack and blister in service.

Yet another object is to provide carbon or graphite or tungsten orrefractory carbide nozzle inserts embodying the features and advantagesof the foregoing objects.

A still further object is to provide nozzles in accordance with thelast-named object having carbon or graphite as the refractory material,silicon carbide as the reinforcing infiltrant, and silver as theevaporating infiltrant.

Still another object is to provide nozzle inserts for use in rockets ofthe solid-propellant type.

Other objects will be recognized from the following specification.

The invention will be described with reference to the accompanyingdrawing which shows a vertical sectional view through a rocket nozzleinsert in accordance with the invention. It comprises a porous body 1 ofrefractory material such as graphites, a refractory metal such astungsten, or refractory metal carbides. Body 1 has a Venturi-type boresuch as is shown in Pat. 3,145,529, the surface of which provides aflame-contacting face 2 for the hot propellant flame which passesthrough the bore in the direction of arrows 3. The outside surface ofthe body 1 constitutes a back, or outer, face. The microporosity of theinsert in indicated by stippling 4. In accordance with the invention,the insert is provided with an array of closely spaced small diameterpassages 5 laterally disposed relative to the axis of the nozzle andwhich extend from the flame face to the back face.

In accordance with this invention its objects are attained by porousnozzle inserts of the types and materials described above having anarray, or multiplicity, of closely spaced, small diameter passagesradially disposed relative to the axis of the nozzle and extending fromthe flame face to the back face. The said passages are infiltrated withan evaporable coolant or coolants.

My novel throat inserts are made, as heretofore, by practices known inthe art of carbon and graphite manufacture, and in the art of powdermetallurgy. The passages which characterize this invention are formed inthem in any desired manner, and the microor macroporosity 4 and thepassages 5 are infiltrated in known ways. Various practices may, ofcourse, be used such as those applied to the production of graphitearticles in general, or to the production of tungsten articles orcarbide articles from powdered materials. Thus, a conventional hollowcylindrical billet of tungsten powder is isostatically pressed andpresintered, drilled with the desired multiplicity of passages, andsintered to a density of 70- 80% of full density; or a conventionalcarbon rnix containing binder is extruded, molded, or isostaticallymolded into a hollow cylindrical billet, baked, for instance at 2000 F.with formation of an internal porosity of from 4 10 to 40% of thevolume, graphitized, for instance at 4500" to 5000 F., then drilled withthe desired multiplicity of passages.

Infiltration of the microporosity 4 and of the passages 5 may beaccomplished after the aforementioned steps. Although a singleevaporative infiltrant may be used for both the passages and the pores,it has been found that much of an evaporative coolant in the pores nearthe back face is not effective for that function. In accordance with theinvention I prefer, accordingly, to infiltrate the fine pores in theregion of the back face only with a reinforcing agent to minimize thedanger of cracking as a result of thermal and flame-pressure stressescreated during firing.

Preferably this is accomplished by first infiltrating the finestporosity to a depth of a tenth to a quarter of the thickness of theinsert at its throat, measuring from the back face, with a material M oflow expansivity, most suitably one which is non-volatile at the maximumtemperature reached by the back face during the mission and remainssolid during the use of the nozzle. This infiltrant should have a lowcoeflicient of thermal expansion; it should have a melting point and aboiling point higher than those of the autotranspiring infiltrant, andits surface tension and interfacial tension with respect to therefractory material should both be such that it will infiltrate the finepores spontaneously, or under moderate pressure. Some glasses, andmetals such as chromium, columbium, iridium, molybdenum, osmium,zirconium, and silicon and some of their alloys, fit these requirements.Especially suitable for this purpose if the refractory is graphite arelithium-silicon alloys and calciumsilicon alloys, especially lithium andcalcium disilicides, as disclosed in application Ser. No. 519,945, filedJan. 11, 1966, by John C. Kosco. In the molten and superheated statethose materials will infiltrate spontaneously to the desired depth andare then converted to silicon carbide, with evolution of the calcium orlithium as gases, by subsequently heating to a higher temperature. Forexample, the infiltration may be done at 2000 F. and the subsequentconversion at 2500 F. Or there may be used one of the other metals orglasses mentioned, in conjunction with pressure or with wetting alloyingagents that will cause them to infiltrate graphite spontaneously. If therefractory material is tungsten or a carbide, a similar method known inthe art may be utilized, taking into account their different abilitiesto be infiltrated spontaneously. This infiltrant, being solid andnon-volatile at the maximum backface service temperature, increases theresistance of the refractory material to the stresses due todifferential thermal expansion which arise during firing.

The autotranspiring infiltrant, or, if there are more than one, at leastone of them, should have a melting point above 1000 F., a boiling pointof at least about 2100 F. and preferably between 3000 and 5000 F., ahigh vapor pressure at the operating flame temperature, say 5500 F. ormore, a high latent heat of vaporization, and a low thermal expansioncoeflicient. Likewise, this infiltrant should have a high enough surfacetension and a low enough interfacial tension with the refractorymaterial to permit it to be completely infiltrated, with only moderateapplication of pressure, if any, into the passages 5 and the porosity 4of the refractory material remaining open after the first infiltrationto provide a body of the autotranspiring infiltrant. Furthermore, theautotranspiring infiltrant should not react with the refractory materialat any temperature below at least 4500 F. For missions of mediumseverity silver, copper, and barium fit these requirements reasonablywell. For missions of high severity the passages are infiltrated with ametal meeting the characteristics stated, the voids resulting from itssolidification then being infiltrated with a lower melting and highboiling metal examples of which are lead, magnesium and zinc to providea zone containing both the high melting point and the lower meltingpoint autotranspiring metals.

The evaporation of the evaporating infiltrant from the passages andbores during service can lower the temperature at the flame face of thenozzle by 400 to 800 F., with consequent lowering of the stresses at theback face, as is desirable for increasing the resistance of therefractory material at the flame face to erosion, softening, andblistering, and to prevent cracking at the back face. The filling of thepores near the back face with silicon carbide or other reinforcingmaterial can raise the resistance to cracking at this face, where cracksare commonly initiated, by a factor of two or three.

The passages which characterize this invention provide, as anadditional, but minor function, paths of highly conductive metal alongwhich heat can flow rapidly to the back face, thus decreasing to someextent the thermal gradient which tends to cause cracking, but, moreimportantly, they provide paths of larger diameter than the fine poreswhereby to improve release of the evaporating infiltrant and thus toimprove cooling efficiency. This improved release of the evaporatinginfiltrant occurs in two ways: first, directly, by providing such largerpaths for the infiltrant that is originally in the passages themselves,and, secondly, by providing a much shorter smalldiameter path followedby a longer and relatively larger diameter path for the vaporevaporating from the pores in the material between passages.

As exemplifying the benefits of the passages which characterize thisinvention, inserts with passages inch in diameter on inch centersincrease the evaporative release of silver by about 33 percent incomparison with a similar nozzle insert without the passages, eachhaving pores averaging two microns in diameter. As the passages aredisposed closer together, the comparative increase in release rate goesup rapidly. For instance, with inch diameter passages on inch centersthe release of silver in one minute approaches 90 percent of thatpresent in inserts 1% inch thick at the throat, with a flame temperaturenear 6000" F., while it is only approximately 50% without the passages.Although the presence of the infiltrant-filled passages does weaken therefractory material to a minor extent, the elfect of the extra coolingin lowering the stresses at the back face more than offsets this minorweakening. Thus it is evident that not only does the use of passages inaccordance with my invention decrease stress and improve reliability,but also such nozzles permit the use of propellants of higher flametemperature than nozzles without the passages.

In high severity tests of nozzles according to the invention the bestresults with graphite were had with Stackpole Carbon Co. Grade 2000electrographite (porosity 13.5%) infiltrated with copper and then leador with silver and then lead, while the best results with tungsten werehad with metal of 20 percent porosity and infiltrated with silver.

Nozzle inserts of graphite possess various advantages. Thus, incomparison with tungsten and other refractory metals they are of lighterweight, which is advantageous for missile purposes. Too, graphite is notsubject to the surface sintering with its undesirable consequencesalluded to above. Despite its outstanding properties at hightemperatures, especially strength, the idea of using graphite has oftenbeen rejected for use as nozzle inserts because of poor low temperaturestrength. Thus, firing stresses reach a maximum when the back face isstill relatively cool, thus posing a problem of low temperaturestrength. However, this problem becomes of minor importance with theinserts of this invention which are strength-reinforced as describedabove. Experience has shown that graphite inserts of this invention,infiltrated with silver, or with copper, or with one of these metals andlead, are as satisfactory for some high-performance missions as issilver-infiltrated tungsten in regard to resistance to firing stresses,erosion and 6 corrosion, and are far superior in regard to weight andcost.

It will be understood that porous carbon, graphite, tungsten, orrefractory-carbide bodies having an array of infiltrated passages asdescribed above may be treated in similar manner for handling combustionproducts or hot gases or liquids under other conditions, either more orless severe than prevail with rocket nozzles, as for examples inre-entry bodies, hot-gas deflectors and plumbing in rockets, and nozzlesand plumbing used in metallurgical and chemical processes.

According to the provisions of the patent statutes, I have explained theprinciples of my invention and have described what I now consider torepresent its best embodiment. However, I desire to have it understoodthat, within the scope of the appended claims, the invention may bepracticed otherwise than as specifically described.

I claim:

1. A rocket nozzle or rocket nozzle insert having a flame-contactingface and a back face comprising a shaped body of refractory material ofthe group consisting of carbons, graphites, tungsten, and refractorycarbides, said body being porous and provided with a multiplicity ofclosely spaced, radially disposed, smalldiameter passages extendingbetween said faces, the porosity and passages being infiltrated in theregion of said back face to a depth between one-tenth and one-quarter ofthe thickness of the article at its throat with a substance that issolid, non-volatile, and of low coeflicient of thermal expansion at theoperating temperature of the back face of the nozzle insert and actingto reinforce the article against stresses due to dilferential expansionbetween the said faces, and the porosity and passages then remainingopen being infiltrated with a metal melting above about 1000 F.,exerting high vapor pressure at the flame temperature and beingnon-reactive with the refractory material at the temperatures the laterreaches in service.

2. An article according to claim 1, the porosity being infiltrated inthe region of said back face to a depth between one tenth and onequarter of the thickness of the article at its throat with a substancethat is solid, nonvolatile, and of low coefficient of thermal expansionat the operating temperature of the back face of the nozzle insert andacting to reinforce the article against stresses due to differentialexpansion between the said faces.

3. An article according to claim 1, said substance being siliconcarbide, and said refractory material being graphite.

4. An article according to claim 1, said body being graphite, saidpassages being infiltrated with silver, the porosity in the region ofsaid back face being infiltrated to a depth between one tenth and onefourth of the thickness of the article with silicon carbide, and theremaining porosity of the body being infiltrated with silver.

5. An article according to claim 1, said body being graphite, saidpassages being infiltrated with copper, the porosity in the region ofsaid back face being infiltrated to a depth between one tenth and onefourth of the thickness of the article with silicon carbide, and theremaining porosity of the body being infiltrated with copper.

References Cited UNITED STATES PATENTS 3,069,847 1'2/1962 Vest 6020O3,137,995 6/1964 Othmer et al. 239-265.15 3,145,529 8/1964 Maloof 60-2003,253,405 5/1966 Kropa 239265.15

ALLEN N. KNOWLES, Primary Examiner US. Cl. X.R. 60200

